1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to a technology for transmitting downlink Hybrid Automatic Repeat request (HARQ) information in a wireless communication system supporting an HARQ technique.
2. Description of the Related Art
A wireless communication system applies an HARQ technique as one of technologies for improving data reception performance. As a representative example, a Long Term Evolution (LTE) system which is the next generation mobile communication system of an asynchronous cellular mobile communication standard organization 3rd Generation Partnership Project (3GPP) uses an HARQ technique for downlink and uplink data transmission.
FIG. 1A illustrates an ACK/NACK transmission structure in an LTE system according to a conventional art.
Referring to FIG. 1A, a subframe 100 having a length of 1 ms, which is a basic unit of downlink transmission in an LTE system consists of two slots 101 of 0.5 ms. Assuming a general Cyclic Prefix (CP) length, each slot consists of seven Orthogonal Frequency Division Multiplexing (OFDM) symbols 102. Also, a Physical Resource Block (PRB) 103 consists of 12 subcarriers in the frequency domain, and as a resource allocator corresponding to one slot, the PRB consists of a resource element 104 corresponding to one subcarrier and one OFDM symbol in the time domain. Here, FIG. 1A illustrates an ACK/NACK transmission structure assuming that a base station cell supports two antenna ports and a control region has been set to three OFDM symbol sections. A PRB illustrated to the left illustrates an ACK/NACK transmission structure for a first antenna 105, and a PRB illustrated to the right illustrates an ACK/NACK transmission structure for a second antenna 106.
As described above, a resource region for downlink in an LTE system is roughly divided into a control region and a data region. First, the control region means a communication resource including a Physical Downlink Control Channel (PDCCH) for transmitting control information for each terminal, and a Physical HARQ Indicator Channel (PHICH) for transmitting ACK/NACK information for uplink data. The control region may be freely allocated within a first OFDM symbol section to a third OFDM symbol section. Next, the data region means a series of communication resources including a data channel (Physical Downlink Shared Channel (PDSCH)) for transmitting data such as voice and a packet transmitted to each terminal. Also, the LIE system allows all terminals inside a cell to estimate a downlink channel by transmitting a Common Reference Signal (CRS) via each of the data region and the control region.
A conventional LTE system allocates a PHICH for transmitting ACK/NACK information for uplink data to the control region, and performs dispersion transmission throughout available frequency band. That is, the conventional LTE system generates an ACK/NACK signal for each terminal as a BPSK symbol, and then spreads it to four symbols by means of a Walsh code, and maps the four symbols to four successive resource elements. Therefore, one ACK/NACK signal is mapped to the four successive resource elements, and the four successive resource elements to which one ACK/NACK signal has been mapped are referred to as a Resource Element Group (REG). Additionally, an IQ multiplexing scheme may be applied, so that eight PHICH ACK/NACK signals may be multiplexed per REG at the maximum. The REG configured in this manner is generated repeatedly three times, and the repeatedly generated three REGs are scattered with a distance from one another on a frequency within the control region of all PRBs forming an available frequency band. At this point, a resource index for a Walsh code allocated for PHICH transmission for each terminal and an REG, etc. is determined by a function that uses a plurality of parameters such as a lowest index value among PRB indexes allocated during Physical Uplink Shared Channel (PUSCH) transmission, a circulation transition field value of a Demodulation RS (DM RS) within a PDCCH including a most recent uplink DCI format, etc. as an input. Therefore, a terminal may implicitly discriminate a PHICH related resource for the terminal itself using the above-described plurality of parameters.
As described above, the conventional LIE system transmits ACK/NACK information for uplink data using all PRBs forming a frequency band. However, since it is considered that a method of transmitting a PHICH using all PRBs forming a frequency band consumes a resource seriously because it should transmit a Reference Signal (RS) over an available entire frequency band, a PHICH transmission technology capable of reducing resource consumption is required. Furthermore, to obtain more improved PHICH reception performance, a PHICH transmission scheme suitable for various multi-antenna transmission techniques needs to be provided. Also, for more swift PHICH transmission in a PHICH transmission scheme applying a multi-antenna transmission technology, a feedback method needs to be provided.